An annular solar eclipse occurs when the moon is directly between Earth and the sun but, because the moon is at one of the farther points from Earth on its orbit, it's too small to completely cover the sun.
Credit: Daniel Fischer
If you plan to be anywhere from the southern third of Africa, to southern and eastern India, southeastern China, Southeast Asia, and virtually all of Australia on Monday, January 26, you will be treated to a view of a solar eclipse.
This will be an annular or ring eclipse of the sun, so called because the moon's disk will appear too small to completely cover the sun's disk. This circumstance is due to the fact that the moon will be a bit farther from Earth than average.
An annular eclipse, though a rare and amazing sight, is far different from a total solar eclipse. Since the sun's brilliant surface or photosphere never completely disappears, the panoply of striking phenomena seen during a total eclipse such as the corona and prominences and the dramatic darkening of the sky accompanied by some of the brighter stars and planets, will not be seen.
Rather, at maximum, skywatchers will see a "penny atop a nickel" effect, with the sun mimicking a blazing ring of light rimming the dark silhouette of the moon (creating the so-called "annulus" or ring effect). Put another way, the dark cone of the moon's shadow, called the umbra, will not be long enough to reach the Earth. So, anybody who is standing on the patch of Earth to which the umbra points sees up all around it – a ring, or annulus of sunlight surrounding the dark moon.
A strange "counterfeit twilight" falls over the landscape, but it falls far short of even a miniature night; in essence, this is really nothing more than a very fancy partial eclipse.
Where it's visible
The moon's faint outer shadow or penumbra first touches Earth at dawn in the South Atlantic Ocean about 500 mi. (800 km.) off the South African coast. A bit over an hour later, the central axis of the shadow – the so-called "antumbra" – arrives, in the South Atlantic about 1,600 mi. (2,600 km.) west of southern Namibia, Africa.
From here, the path of the annular eclipse curves east-southeast, completely missing southernmost Africa. As seen from Cape Agulhas, the geographic southern tip of the African continent, the moon will appear to cover 74 percent of the sun's diameter as the antumbra passes a few hundred miles to the south.
Unfortunately, virtually the entire annularity track falls over open waters, sliding first over the South Atlantic, and then tracking east, then northeast over the Indian Ocean. The eclipse reaches its maximum at local apparent noon, pretty much over the middle of the Indian Ocean. A shipboard observer would see the moon's dark outline wholly inside the sun for 7 minutes and 54 seconds, changing the sun into a "ring of fire."
The first landfall by the antumbra finally comes in the late afternoon about half way from Australia to Sri Lanka over the Cocos Iand Keeling Islands, which belong to Australia and are thickly covered with coconut palms and other vegetation.
As the path of annularity approaches its end, it sweeps northeast through Indonesia, and in the process crosses over portions of Sumatra, Java, Borneo, Malaysia and Sulawesi, before finally coming to its end at local sunset over the Celebes Sea. Yet, the antumbra somehow manages to avoid all of the major population centers in this part of the world. Its northern limit will slide just to the southeast of Palembang (Sumatra) and its southern limit will even more narrowly miss Jakarta (Java), which currently is the eleventh largest city, fifth largest metropolitan area and ninth most densely populated city in the world. Taking a brief jog across the Java Sea, the shadow crosses over into Borneo, the southern limit of the antumbra passing just to the north and west of Balikpapan and its 500,000 inhabitants.
For all of these cities, at maximum eclipse, the sun will not appear as a ring; rather it will mimic a horseshoe with pointed tips.
Outside the path of annularity, a partial eclipse will be visible. The closer you are to the annular track, the larger the "bite" that the passing moon will appear to take out of the sun. In Australia, the eclipse falls on Australia Day; a National holiday. Much of that country (except Tasmania) will get a glimpse of the eclipse coinciding (or nearly so) with local sunset.
A listing of local circumstances for selected locations within the eclipse zone can be found here.
To look at the sun without proper eye protection is dangerous. Even if you are in the path of the annular eclipse you will need to protect your eyes.
By far, the safest way to view a solar eclipse is to construct a "pinhole camera." A pinhole or small opening is used to form an image of the sun on a screen placed about three feet behind the opening. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the sun onto a white card. Just be sure not to look through the binoculars or telescope when they are pointed toward the sun!
A variation on the pinhole theme is the "pinhole mirror." Cover a pocket-mirror with a piece of paper that has a quarter-inch hole punched in it. Open a sun-facing window and place the covered mirror on the sunlit sill so it reflects a disk of light onto the far wall inside. The disk of light is an image of the sun's face. The farther away from the wall is the better; the image will be only one inch across for every 9 feet from the mirror. Of course, don't let anyone look at the sun in the mirror.
Acceptable filters for unaided visual solar observations include aluminized Mylar. Some astronomy dealers carry Mylar filter material specially designed for solar observing. Also acceptable is shade 14 arc-welder's glass, available for just a few of dollars at welding supply shops.
Unacceptable filters include sunglasses, color film negatives, black-and-white film that contains no silver, photographic neutral-density filters, and polarizing filters. Although these materials have very low visible-light transmittance levels, they transmit an unacceptably high level of near-infrared radiation that can cause a thermal retinal burn. The fact that the sun appears dim, or that you feel no discomfort when looking at the sun through the filter, is no guarantee that your eyes are safe.
Later this year . . .
On July 22, the longest total solar eclipse in the 21st century will take place, as the moon's dark umbral shadow races across central India, the People's Republic of China (including Shanghai), and some of the Ryukyu Islands of Japan. At the point of greatest eclipse, totality will last 6 minutes 39 seconds; the longest of any eclipse between 1991 and 2132; it is an eclipse that is eagerly being awaited by millions who live along the path of totality and countless thousands more from around the world who plan to position themselves within the path.
- Gallery: Total Solar Eclipse
- More Images: Rare Solar Eclipses
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.